Error Minimization in Profile Data-Collection Methods 



Error types 



24. The biases and errors of the CRAB-Zeiss method can be broken down 

 into two distinct categories: limitations of the instrument and operator 

 errors . 



25. The Zeiss Elta-2 is capable of measuring both horizontal and verti- 

 cal angles to +0.6 sec of arc and distances to +1 cm in the mode most commonly 

 used at the FRF. The specified range when using a triple prism configuration 

 is 2 km. Increasing the number of prisms increases the range. 



26. The instrument is sensitive to atmospheric and climatic varia- 

 tion since it uses the speed of light to determine distance and optical aiming 

 to measure the angles. The instrument does allow for rough adjustment for 

 these variables. During the summer months, heat shimmer and temperature grad- 

 ients near the land-sea interface may also affect the accuracy of the angular 

 measurements. These errors are assumed to be negligible relative to other 

 types of errors. 



27. On one occasion the Elta-2 developed an internal problem which 

 introduced a gradual bias into the data. This bias was identified and removed 

 during processing. Errors of this type are difficult to remove because they 

 occur radially from the instrument's position and do not uniformly affect the 

 data. The instrument was repaired, and the malfunction did not reoccur. 

 Treatment of errors in the data is discussed below. 



28. Operator errors can result from improper leveling of the instrument, 

 an error in positioning the instrument in coordinate space (particularly in 

 elevation), movement of the tripod during the survey, and mis-aiming the 

 instrument at the proper location on the prism cluster while taking the mea- 

 surement. Constructed examples of typical nearshore survey errors and their 

 impact are shown in Figure 7. All the measured points are affected by im- 

 proper leveling or improper location in coordinate space. Individual points 

 are affected by incorrect aim. 



29. A different type of operator error results when topographically 

 important points, such as the bar crest or trough, are missed. Survey points 

 are selected based on the timed travel of the CRAB. More points are taken 

 close to shore where the profile is more complex. It is possible for the 



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